Are you using the big fixed transponder or the small one?

I bought a small one from CVS earlier this year, register my rental car online as soon as I get it ... the first car I got was from Sixt, had no issues, first couple of tolls read the transponder no problem, second two tolls were from the license plate, got read and billed to my account no problem.

Was in Florida again a few weeks ago, registered my Hertz car as soon as I picked it up as a "Rental Car" on the website, even now the website shows I registered the car at 3:02 PM or so. A month later I got a bill from PlatePass (a division of everyone's favorite, American Traffic Solutions) for $16.10, $4.95/day plus $1.25 for the full price of a single toll, about 20x the amount it would have been for me to pay it on my SunPass. Look at the details on the PlatePass website and it was my rental car at about 3:11 PM.

I've read elsewhere that at best SunPass will only switch the rental car over within 24 hours, so even though the SunPass website shows in my account that plate was registered to me, when they generated the toll from the license plate read it was still sent to American Traffic Solutions. So, use SunPass at your own risk on rental cars.

I sent an email to SunPass to see if they could switch the toll to my account, no go. I've now sent a complaint letter to the Director of Toll Operations or some such title and have not heard back, may need to follow up, because I think it's unacceptable that they can't send the toll to the correct account.